In the most up-to-date radio communication system used in a cellular system for mobile communication such as a cellular phone, or the like, for example, a plurality of antennas are provided to each radio communication station by employing MIMO, and also a study of introduction of CDD (Cyclic Delay Diversity) that is one type of DD (Delay Diversity) as a transmission diversity is advancing.
In the delay diversity, a transmitting station transmits the same signals from a plurality of antennas and such signals are controlled such that a sufficient time difference (delay) in signal is produced among a plurality of antennas respectively. Accordingly, even when an interval between a plurality of antennas is small, a sufficient difference (time difference) is produced in the radio signals that arrive at the receiving station from a plurality of antennas of the transmitting station respectively. Therefore, the receiving station can recognize differences in propagation paths (paths) of a radio wave respectively, and can extract a target signal by separating respective signals every path. As a result, a diversity effect can be achieved.
Also, a delay time is changed cyclically in CDD. For example, as disclosed in Non-Patent Literature 1, when communication is held by using OFDM (Orthogonal Frequency Division Multiplexing) in which a large number of subcarriers whose frequencies are different mutually are superposed, a mutually different cyclic delay is assigned to respective frequencies of the subcarriers such that an amount of delay (or phase) is changed every frequency of the subcarrier.
Accordingly, a frequency selectivity appears, as shown in FIG. 17A, for example, in the propagation characteristic of the radio propagation path that extends from the transmitting station to the receiving station. FIGS. 17A and 17B are graphs showing concrete examples of a frequency characteristic in the radio propagation path and a frequency characteristic of a phase shift respectively. FIG. 17A shows an amplitude on the propagation path in the first-time transmission, and FIG. 17B shows an amplitude on the propagation path in the second-time transmission by the resending operation. In an example shown in FIG. 17A, such a case is assumed that a phase difference (phase rotating angle) φk given by Formula 1 is assigned to the signal of each subcarrier signal as a phase shift, where N denotes an FFT (Fourier Transform) size (the number of subcarriers), D denotes a phase shift amount, and k denotes the subcarrier number.
                              [                      Formula            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                ϕ          k                =                  ⅇ                      j            ⁢                                                  ⁢            2            ⁢                                                  ⁢            π            ⁢                          D              N                        ⁢            k                                              (        1        )            
That is, the frequency characteristic in which a section where a receiving level (amplitude) is large (good receiving condition section) and a section where a receiving level is small (falloff in level: notch) appear periodically at an interval of N/D on the frequency axis is obtained. Therefore, the frequency selectivity on the radio propagation path can be enhanced by employing CDD, and thus a frequency diversity effect can be achieved. The section where the receiving level (amplitude) is large, mentioned herein, corresponds to the section that is derived by summing up the good receiving condition sections out of instantaneous variations of the signals that arrive from the transmission antennas respectively. Thus, SINR (Signal-to-Interference plus Noise power Ratio (Interference Signal Suppression Degree)) for a desired radio wave can be enhanced.
In contrast, when the propagation state in the radio propagation paths between the transmitting station and the receiving station became worse, in some cases the signal transmitted from a target transmitting station (desired wave) cannot be correctly received at the receiving station. In such case, the resending control is applied in many cases. More particularly, the receiving station informs the transmitting station of NACK (Not Acknowledgement) when it could not correctly receive a packet, or the like of a desired wave, and then the transmitting station transmits repeatedly the same data (packet, or the like) as the precedingly transmitted data (the data whose transmission is failed) when it detects NACK from the receiving station.
Even when the propagation state in the radio propagation paths became worse, a chance that the receiving station can receive correctly the target signal is increased by applying the resending control. In particular, when hybrid ARQ (Automatic Request for Repetition) is employed, a decoding of the received signal is tried while utilizing the information of signals that were received in the past. As a result, a probability that the receiving station succeeds in a reception at a time of resending operation is increased.
However, in the circumstances that the propagation state in the radio propagation paths does not so change with the lapse of time within a predetermined frequency width, a chance that the receiving station succeeds in a reception of the packet is not increased even when the resending control is applied. Therefore, consequently the same packet should be transmitted repeatedly in many times, and throughput (an amount of data transmitted per unit time) is lowered. In this manner, due to the event that an amplitude in the propagation path is not changed within a frequency width (in a relative bandwidth) whose variation on the propagation path can be regarded constant, in some cases a synthesized gain obtained by the resending operation becomes small and thus an effect of resending control cannot so much obtained.
Also, an effect of a frequency diversity can be achieved by employing CDD. However, when the propagation state in the radio propagation paths was seldom changed in the resending operation from that in the transmission at the first time, an effect of the resending control cannot so much obtained in the above-mentioned situation. In this event, as shown in FIG. 17(b), a change of the frequency selectivity is not so much produced by CDD between the first-time transmission and the second-time transmission (the resending operation) even when the resending control is applied, and thus a fading falloff due to the frequency selectivity occurs in the same position on the frequency axis. As a result, such a problem existed that a synthesized gain obtained when the resending signals are composed by the resending control is not so much improved by employing CDD, and thus a probability of successful reception at a time of resending operation cannot be so much enhanced.    Non-Patent Literature 1: 3GPP TSG RAN WG1 #42, R1-050715, Motorola, “EUTRA Downlink MIMO Requirements and Design”, 2005